Dynamic control systems for automotive vehicles have recently begun to be offered on various products. Dynamic control systems typically control the yaw of the vehicle by controlling the braking effort at the various wheels of the vehicle. Yaw control systems typically compare the desired direction of the vehicle based upon the steering wheel angle and the direction of travel. By regulating the amount of braking at each corner of the vehicle, the desired direction of travel may be maintained. Typically, the dynamic control systems do not address roll of the vehicle. For high profile vehicles in particular, it would be desirable to control the rollover characteristic of the vehicle to maintain the vehicle position with respect to the road. That is, it is desirable to maintain contact of each of the four tires of the vehicle on the road.
In vehicle roll stability control, it is desired to alter the vehicle attitude such that its motion along the roll direction is prevented from achieving a predetermined limit with the aid of the actuation from the available active systems such as controllable brake system, steering system and suspension system. Although the vehicle attitude is well defined, direct measurement is usually impossible.
Existing yaw stability control systems may aid in preventing a vehicle from spinning out, and hence may indirectly reduce the potential for the vehicle to have a side collision with a barrier thus reducing the likelihood of a rollover. However, due to different control objectives for yaw stability and roll stability, the standard yaw stability control system will not directly or automatically improve vehicular roll stability. Vehicle tests have shown that the standard yaw stability control system does not react properly to many on-road rollover events. One reason is that the yaw stability control system is intended to regulate both the under-steer and the over-steer of the vehicle such that during driving on abnormal road surface conditions the vehicle can still be controlled by a driver using his driving skills developed for normal road surface conditions. The roll stability control system, however, needs to make the vehicle under-steer more during the detected aggressive driving conditions that may contribute to vehicle roll instability. Intentionally making the vehicle under-steer (as required for roll stability control) and intentionally making the vehicle neutral-steer (as required for yaw stability control) are two different objectives. Notice, however, that if a near-rollover event is caused by an aggressive over-steer, the yaw stability control system might help improve roll stability due to the fact that it brings the vehicle to neutral-steer so as to reduce the amount of vehicle over-steer.
It is therefore desirable to provide an enhanced yaw stability control system such that the traditional yaw stability function is preserved and at the same time the system will directly and properly react to potential vehicular rollover events.